There is a well recognized need in the electronics industry for a rechargeable energy source with high reliability that can provide high power, that can be charged, discharged, and recharged quickly, and that has a high life cycle, i.e., is long life. Among applications that could benefit from such as device are industrial applications, consumer applications, and automotive applications.
Double layer capacitors, also referred to as electrochemical capacitors, are energy storage devices that are able to store more energy per unit weight and unit volume than traditional capacitors. In addition, they can typically deliver the stored energy at a higher power rating than rechargeable batteries.
Double layer capacitors consist of two porous electrodes that are isolated from electrical contact by a porous separator. Both the separator and the electrodes are impregnated with an electrolyte solution. This structure allows ionic current to flow between the electrodes through the porous separator at the same time that the porous separator prevents an electrical or electronic current (as opposed to ionic current) from shorting the two porous electrodes. Coupled to the back of each of the active electrodes is a current collecting plate. One purpose of the current collecting plate is to reduce ohmic losses in the double layer capacitor. If these current collecting plates are non-porous, they can also be used as part of the capacitor seal.
Double layer capacitors store electrostatic energy in a polarized liquid layer which forms when a potential exists between two electrodes immersed in an electrolyte. When the potential is applied across the electrodes, a double layer of positive and negative charges is formed at the electrode-electrolyte interface (hence, the name “double layer” capacitor) by the polarization of the electrolyte ions due to charge separation under the applied electric field, and also due to the dipole orientation and alignment of electrolyte molecules over the entire surface of the electrodes.
A major problem in many carbon electrode capacitors, including double layer capacitors, is that the performance of the capacitor is often limited because of the high internal resistance of the carbon electrodes. This high internal resistance may be due to several factors, including the high contact resistance of the internal carbon-carbon contacts, and the contact resistance of the electrodes with a current collector. This high resistance translates to large ohmic losses in the capacitor during the charging and discharge phases, which losses further adversely affect the characteristic RC (resistance×capacitance) time constant of the capacitor and interfere with its ability to be efficiently charged and/or discharged in a short period of time. There is thus a need in the art for lowering the internal resistance, and hence the time constant, of double layer capacitors.
Another area of concern in the fabrication of double layer capacitors relates to the method of connecting the current collector plate to the electrode. This is important because the interface between the electrode and the current collector plate is another source of internal resistance of the double layer capacitor, and such internal resistance must be kept as low as possible.
Recently, electrochemical capacitors employing non-aqueous (organic) electrolyte solutions have been developed. These double layer capacitors have the advantage of higher operating voltage, but generally suffer from higher internal resistance. Nonetheless, the operating voltage greatly increases the energy density of the double layer capacitor. For example, an aqueous double layer capacitor may only operate at 0.67 volts per cell, while a similar non-aqueous device will operate at 2.3 volts per cell. This difference in operating voltage increases energy density by a factor of 11.8.
Unfortunately, non-aqueous electrolytes tend to be much more sensitive to impurities, such as water or oxygen, in the electrolyte. Any level of these impurities will lead to gas generation within the double layer capacitor when the double layer capacitor is operated at high voltage. Because of this, manufacturers of non-aqueous electrolyte double layer capacitors take great care is limiting the levels of water and oxygen contamination within the electrolyte solution during manufacture, striving to achieve levels of contamination on the order of 10 to 100 parts per million.
In order to achieve long life in a double layer capacitor employing a non-aqueous electrolyte and that is operated at high voltage, care must be taken in limiting the influx of water and oxygen into the electrolyte solution. Commercially available non-aqueous electrolyte double layer capacitors are packaged with sealing technologies that limit the life of these double layer capacitors due to the influx of water and oxygen.
Another issue that continues to face virtually any technology that involves electronics is that of miniaturization. With smaller and smaller devices being designed, and thus smaller and smaller components being demanded, pressures have been put on the makers of double layer capacitors to decrease device size, while maintaining a high level of capacitance. This demands not only extremely low internal resistances, but poses an additional problem.
This additional problem lies in the fact that, at least in non-aqueous electrolyte double-layer capacitors, environmental contamination from, for example, air and water leaking into the electrolyte result in a significant reduction in capacitance, and a corresponding increase in resistance, namely resistance to ionic current flow.
While in conventional double-layer capacitor devices conventional technology has been applied to seal the capacitors so as to both contain the electrolyte and to prevent contamination of the electrolyte with oxygen and water, as devices become increasingly miniaturized, conventional techniques are no longer suitable. Further complicating this problem is the fact that materials used in sealing the double-layer capacitor must be thermally and chemically compatible with the electrolyte and the case of the capacitor, and provide appropriate electrical characteristics, i.e., be conductive or an insulator, depending on where the seal is positioned.
With small double-layer capacitor devices, a further complication arises in that consumer demand is for devices that can be directly soldered to printed circuit boards. Thus external terminals must not only be compatible with the sealing approach adopted (and it with them), but must be of a material that is solderable using conventional soldering techniques and materials. Furthermore, in a conventional automated soldering process, the case and internal components of the double layer capacitor must be able to withstand exposure to high temperatures, for example, temperatures of up to 250 degrees Celsius for up to 5 minutes.
It is thus apparent that there is a continuing need for improved double layer capacitors. Such improved double layer capacitors need to deliver large amounts of useful energy at a very high power output and energy density ratings within a relatively short period of time, and need to be produced in a small, solderable, long-life device. Such improved double layer capacitors should also have a relatively low internal resistance and yet be capable of yielding a relatively high operating voltage. It is also apparent that these devices should be of low internal resistance.